1. Field of the Invention
The present invention relates to a process and catalyst for the direct synthesis of dimethyldichlorosilane.
2. Description of the Prior Art
The industrial process for the production of organochlorosilanes, and especially dimethyldichlorosilane (hereinafter referred to as "DMCS"), is a well-known process which is particularly described in U.S. Pat. No. 2,380,995, and in the text by Walter Noll, Chemistry and Technology of Silicones, pp. 26-41, published by Academic Press Inc. (1968).
According to this process, known to this art as "direct synthesis" or "Rochow synthesis", organochlorosilanes, and in particular DMCS, are produced directly by reacting methyl chloride with solid silicon in the presence of copper as a catalyst, according to the reaction: EQU 2 CH.sub.3 Cl+Si.fwdarw.(CH.sub.3).sub.2 SiCl.sub.2.
In actual fact, other products are formed during the direct synthesis, especially CH.sub.3 Cl.sub.3 Si (hereinafter referred to as "MTCS") and (CH.sub.3).sub.3 SiCl (hereinafter referred to as "TMCS").
Other by-products are also formed, such as, for example, CH.sub.3 HSiCl.sub.2 and (CH.sub.3).sub.2 HSiCl, and heavy products which are polysilanes, principally disilanes.
Among all of the products obtained by direct synthesis, DMCS is the product in greatest demand. After hydrolysis and polymerization, this compound permits the preparation of oils and gums which are useful starting materials for the manufacture of silicones. Thus, DMCS is used for the preparation of polyorganosiloxane resins, as described in U.S. Pat. Nos. 2,258,218 to 2,258,222, for the preparation of oils, as described in U.S. Pat. Nos. 2,469,888 and 2,469,830, and for the preparation of polyorganosiloxane elastomers, as described in U.S. Pat. No. 2,448,756.
It is also known to this art to use copper or chemical compounds of copper as a catalyst for the direct synthesis reaction, whether employed in the form of an alloy or a mechanical mixture with silicon, optionally deposited onto an inorganic carrier or support.
Addition of various additives to the copper, in order to improve the yield of DMCS, has already been proposed to this art. These additives can be zinc or a zinc halide (U.S. Pat. No. 2,464,033), aluminum (U.S. Pat. Nos. 2,403,370 and 2,427,605), tin, manganese, nickel and silver (British Patent No. 1,201,466), cobalt (British Patent No. 907,161), and potassium chloride (Russian Patent No. 307,650).
The option of using tin and antimony as an additive to the copper for controlling the reaction has already been mentioned in French Patent No. 950,448.
French Patent No. 1,037,183 also describes the addition of calcium, magnesium, beryllium, strontium and barium as a deoxidizing agent for the contact masses employed in the direct synthesis.
There is no doubt that the aforesaid processes employing these additives enable the direct synthesis process to be improved, but they nevertheless have at least one of the following disadvantages:
(1) The selectivity for DMCS, assessed by the average weight ratio MTCS/DMCS, and/or by the molar percentage of DMCS relative to the total amount of silanes obtained, remains inadequate;
(2) The initiation time and the initiation temperature of the reaction are too high;
(3) The mean activity of the catalyst system, also referred to as the efficiency, assessed as the weight of methylchlorosilanes (MCS) produced per hour and per kg of silicon added, and the maximum silicon conversion remain inadequate;
(4) The catalyst system is sensitive to impurities;
(5) The formation of by-products and especially of disilanes remains high.